1. Field of the Invention
The present invention relates to advancements in measurement of temperature of molten metal and the like and, in particular, to advancements in immersion-type sensor for measurement of temperature of molten metals and more particularly to an optical fiber based temperature sensor. Importantly, the sensor of the invention is directed to a user friendly and cost effective discrete-time temperature measurement of molten metal. Advantageously the invention directed towards the development of a temperature sensor device, which is invulnerable to external electromagnetic radiation, is cost effective and not prone to changes in material emmissivities and can be used to measure the temperature of the molten metal irrespective of the nature of the metal.
2. Description of the Related Art
The knowledge of liquid metal temperature is essential as it directly influences the quality of the end product as well as the productivity of the metal plant. To measure the molten metal temperature, different immersion type temperature measurement techniques are used. Amongst all these techniques, optical temperature measurement and thermoelectric temperature measurement are widely used.
Traditionally, the immersion type thermoelectric temperature sensors are based on thermocouples but these are not viable in several applications. Specifically, they are not immune to electromagnetic radiation, have slow response times and allow for low sampling frequencies. The use of non-contact pyrometry for the measurement of steel temperatures is a known technique and several patents cover various details of this technique—for. e.g. U.S. Pat. No. 4,462,698, U.S. Pat. No. 5,769,540 etc. The major drawback of non-contact pyrometry is that the incident radiation is received not only from the hot body to be measured but other hot bodies that are invariably present in the vicinity. These bodies have different emissivities and introduce complex errors that are difficult to quantify and eliminate. In general, non-contact pyrometry techniques have high errors and some might be outside the tolerance of those acceptable to the industry.
It is also known in the art to involve contact/immersion type temperature sensors wherein instead of picking up radiation from the surface of a hot body and analyzing it for temperature, a probe containing an optical fiber is immersed into the metal. U.S. Pat. No. 6,004,031 and U.S. Pat. No. 6,357,910 B1 describe a similar technique for temperature measurement wherein an optical fiber in reel form is carried. One end of the reel is immersed into the molten metal and the other end is connected to a radiation thermometer, which is essentially an optical pyrometer. However, as a consequence of using the optical fiber in a reel form a length of fiber equal to the thickness of the slag is always wasted on each immersion. This causes losses of the expensive optical fiber during each measurement.
A second problem associated with the presently available contact/immersion type measurements is that when the tip of the optical fiber which is metal-covered is kept immersed into the molten steel for a long time after detecting the temperature of the molten steel, the metallic coating layer of the optical fiber becomes gaseous in the high temperature environment. Then, the generated gas is ejected from the tip of the optical fiber, and is ignited if oxygen is present. To prevent such an accident, the tip of the metal-covered optical fiber is drawn up from the molten steel immediately after measuring the temperature of the molten steel. Then, the tip once used as the temperature measuring element is cut off before next temperature measurement cycle, and the fresh tip is immersed into the molten steel at the next measuring cycle.
The third drawback is that the signal from the hot body is significantly attenuated along the length of the fiber. Thus, the signal received by the radiation thermometer is weak and the signal to noise ratio is low. In addition, the attenuation is dependent non-linearly on the length and since the length of the fiber keeps changing, continuous adjustment has to be made for this changing attenuation. This introduces errors into the measurement.
The fourth limitation is that the above discussed techniques are designed to be continuous measurement techniques and are not suitable in areas where only instantaneous temperature at critical instances of time is required. By using the continuous techniques, the user is forced to measure the temperature continuously over the entire time range and even during phases when the temperature is non-critical. This leads to economic inefficiency
Apart from the above constructional limitations of the present available measuring systems, it is also experienced that such systems also have some limitations in achieving accurate and faster measurement of temperature. The typical techniques used for temperature measurement by radiation pyrometers in the industry are one-color and two-color pyrometry. In one-color pyrometer, the wavelengths are not differentiated and the entire radiation incident on the pyrometer is measured and correlated to temperature using the Stefan-Boltzmann law. In two color pyrometry, two wavelengths are selected and the intensities of the radiation at these two wavelengths is measured and correlated to temperature by using the Planck's Law. However, Planck's Law is a generic correlation between the temperature of the hot body and the wavelength and intensity of the emitted radiation. By reducing the Planck's Law to only 2 wavelengths, a drastic simplification is effected. An inherent assumption is that the emissivity of the body does not change with wavelength. Multi-wavelength pyrometry eliminates the 2-color pyrometry assumptions. In addition, any chemistry effects on the spectrum are eliminated. Multi-wavelength pyrometry is a known technique and U.S. Pat. No. 6,357,010 B1 discloses a method and apparatuses for measuring the temperature of a radiating body utilizing multi-wavelength pyrometry techniques. However, on account of the quantity of data to be handled to accurately predict the temperature, multi-wavelength pyrometry tends to involve intense, time-consuming processing steps.
Thus all such known techniques especially contact type temperature sensor for the measurement of molten metal temperature are either devices which are complex and/or cost extensive to use and hence not convenient for wide scale repetitive use for measurement of temperature of the molten metal and/or have limitations in reaching to the desired accurate and faster determination of temperature of molten metal/steel. There has been thus a need in the art to develop a method and apparatus for molten metal temperature measurement customized for multiple uses and adaptable to any type of metal and which would also be accurate and faster for the purposes of desired temperature determination.